A few concepts of optical physics or mathematics are summarized below. In optics, the luminous exitance of a point of a light-emitting surface is the flux emitted into a half-space by unit area of the emissive surface centered at the point. This concept is sometimes also referred to as emittance or as radiance. A gradient is the derivative along one or more directions of a magnitude, in other words a gradient means that the magnitude varies. A luminous exitance gradient means that luminous exitance varies along the emitting surface. Likewise, a color gradient means that the color of the light emitted varies along the emitting surface. A diffuser is an element made of transparent or translucent material that diffuses light within its bulk and/or at its surface.
The term “singularities” is used to designate small portions of a container or of its surface that present properties that are different from the properties in their neighborhood on or in the container. Optical singularities thus designate portions of a container having optical properties that are different from the optical properties in their neighborhood on or in the container. Specifically, these optical singularities mainly present a refraction and/or reflection effect that is abnormal compared with their neighborhood. Refracting and/or reflecting defects, codes, or indeed decoration such as identification marks on the surfaces of containers thus constitute optical singularities that deflect light in a manner that differs compared with their neighborhood, either in terms of transmission (interfaces) or else in terms of specular reflection.
The term “view” is used to mean an image in the optical sense, i.e. a two-dimensional signal of gray levels or of colors obtained by projecting the container by means of an imaging optical device that is capable of “conjugation” in the optical sense. A camera having a lens can form at least one view. A camera can form a plurality of views simultaneously, e.g. by using a system of mirrors splitting its fields of view into N different views. These views are different if their viewpoints and observation directions are different. It is possible to associate a view with a magnification. In the present application, the term “image” is reserved for designating the signal produced by a camera, where such an image may contain a plurality of views.
Glass is a material that is transparent to visible light. Nevertheless, transmission at each wavelength depends on the tint of the glass and on the thickness passed through by the light in question. The transmittance of glass is measured as a ratio (in %) between the incident light and the transmitted light for a given thickness. It is even possible use a spectrometer to measure transmittance as a function of wavelength so as to obtain a transmission spectrum that characterizes the tint of the glass. For certain low transmittance tints, i.e. tints that are very dark or present a black appearance, and/or for certain thicknesses of glass (walls up to 5 millimeters (mm) thick) with little light passing through the wall of the container, it is possible to have transmittances of less than 1% for practically the entire visible spectrum.
The transparency of glass is a property that is in widespread use for performing optical inspections by making images in transmission of containers and by analyzing those images. The methods in most widespread use consist in backlighting containers using a uniform extended light source, and making images of the containers. Two known phenomena in the field of geometrical optics are used: absorption by so-called opaque defects or by differing optical transmission of the glass (a foreign body or extra thickness of tinted glass), and refraction of light by surface singularities such as cracks, tears, broken seeds, or missing material such as bubbles in the wall, or indeed inclusions of different optical index.
In order to improve the detection of so-called “refracting” defects by reinforcing their contrast in images, U.S. Pat. No. 4,487,322 proposes using a light source presenting a gradient of luminous exitance. Since they deflect light, defects are not illuminated by the same portion of the source as are their neighborhoods, and because of the gradient of luminous exitance in the source, they do not have the same perceived light level as their neighborhoods.
U.S. Pat. No. 6,424,414 describes a method and apparatus for detecting light-refracting defects that might be present in transparent containers. The apparatus described by that patent has a light source presenting a light gradient in a direction along the axis of the container. The light source is in alignment with a camera receiving the light passing through the container. Although such apparatus is suitable for detecting light-refracting defects in transmission, it is not suitable for acquiring images of the container in reflection, in particular for the purpose of analyzing marks or codes made on the surfaces of containers that present low or high transmittance.
For glass containers, it is known to etch a mold number code in molds in the form of beads. When the marks or codes such as mold numbers are made by molding, the portions in relief obtained on the containers are tall (extra thickness greater than >0.5 mm), and these portions in relief (referred to as “beads”) present a size of millimeter order (diameter >0.8 mm).
It is known to read back mold numbers molded on containers by using devices that set each container into rotation and that illuminate the beads by means of directional and focused lightwaves (light beams that are narrow, small, with little or no divergence). Said light rays all having neighboring angles of incidence and they are reflected by the beads in a precise direction defined by the angles of incidence and by the angles of the beads. Sensors pick up the reflections and decode them. Such systems operate only on portions in relief that are of relatively large size and by putting the containers into rotation, which is expensive and disturbs manufacturing lines, which generally involve travel that is linear.
Patent FR 2 780 533 teaches a device using that optical method of reflecting onto the container focused light that has been emitted by a source, but that is adapted to reading in devices that convey containers in translation only, which is much faster and less expensive in use than systems involving rotation. The optical means are adapted to enable an image to be obtained of the entire code-carrying insweep, with the insweep appearing black and the code white. The focused incident light beams are converted to the periphery in the form of a light cone, and the rays reflected by the beads, and by the beads only, are taken by a conical mirror into the form of a plane image in an analysis device. Devices of that type are effective for codes presenting relatively large portions in relief, e.g. taller than 0.5 mm. It is necessary for the beads to have a surface area that is sufficient to be capable of returning a large amount of light in the single observation direction, while the rest of the incident light is reflected in any other direction by the background surface carrying said portions in relief.
In other words, those techniques with directional lighting and a predefined angle of observation work only for portions in relief that are tall, so that the direction in which light rays are reflected by the beads departs sufficiently from the direction of light rays that are reflected by the background surface. It should be observed that that type of system enables the light reflected by the portions in relief to be viewed as white marks compared with a (background) surface that remains black. Furthermore, the high contrast gives information that is practically binary, with beads being seen or not seen, with only bead portions being distinguished, but always at an intensity that is very strong compared with their neighborhoods. When portions in relief are small, e.g. because the molds are worn, the white areas corresponding to the beads in the image becomes small or even not observable.
U.S. Pat. No. 4,644,151 reuses conventional prior art but proposes using a camera having a source that is provided with a luminous exitance gradient in a direction parallel to the axis of rotation of the container. That system thus requires the container to be rotated, giving rise to the numerous drawbacks described above. It is not suitable for reading codes on traveling containers.
When the marks, e.g. in the form of a Datamatrix code, are made by laser on hot glass as described in patent FR 2 907 370, the portions in relief on the surface of glass containers are much smaller than with mold numbers, and in addition the code points are much smaller, having diameters of less than 0.3 mm. The solution consists in obtaining on a priority basis lighting that is uniform and in taking a series of matrix images of the wall of the container while it is rotating. The source is extended and uniform (without gaps), thereby providing the image with a uniform background, but it is of a size that is nevertheless limited in order to make a certain amount of contrast available. That system is thus suitable for marks of the Datamatrix type obtained by laser beam on containers while they are still hot, i.e. having portions in relief that are small. Nevertheless, that system requires the container to be rotated and it is not suitable for reading codes on traveling containers.
The present invention thus seeks to remedy the drawbacks of the prior art by proposing a novel optical technique for observing and analyzing optical singularities that may present only weak refracting power and that are carried by containers.